Well to save time I googled this one. There are a number of sites coming up with solutions. The problem is you have not specified:-
a) the range over which you want the macro to act, just a few rows or the whole sheet, and which few rows exactly ?
b) the interval (N)
c) how many rows to insert at each interval ?
This site came up with one which might suit you as it asks you each time questions a) , b) and c) before it runs.
https://www.extendoffice.com/documents/excel/2993-excel-insert-rows-at-intervals.html
The macro as presented did not actually work because some variables had not been defined. Below is my modified version of their macro.
Option Explicit
Sub InsertRowsAtIntervals()
'Updateby20150707
Dim Rng As Range
Dim xTitleId As String
Dim xInterval As Integer
Dim xRows As Integer
Dim xRowsCount As Integer
Dim xNum1 As Integer
Dim xNum2 As Integer
Dim i As Long
Dim WorkRng As Range
Dim xWs As Worksheet
xTitleId = "Select Range"
Set WorkRng = Application.Selection
Set WorkRng = Application.InputBox("Range", xTitleId, WorkRng.Address, Type:=8)
xRowsCount = WorkRng.Rows.Count
xInterval = Application.InputBox("Enter row interval. ", xTitleId, 1, Type:=1)
xRows = Application.InputBox("How many rows to insert at each interval? ", xTitleId, 1, Type:=1)
xNum1 = WorkRng.Row + xInterval
xNum2 = xRows + xInterval
Set xWs = WorkRng.Parent
For i = 1 To Int(xRowsCount / xInterval)
xWs.Range(xWs.Cells(xNum1, WorkRng.Column), xWs.Cells(xNum1 + xRows - 1, WorkRng.Column)).Select
Application.Selection.EntireRow.Insert
xNum1 = xNum1 + xNum2
Next
End Sub
Ive tested it and it seems to work, but you only need select a column to specify which rows you need the macro to act over.
If its not what you want, there are others out there, some of which have the interval fixed, or may auto detect the range to work over. It depends what you want. let me know if you have problems.